This invention concerns improvements in telephone conference amplifiers.
In known arrangements for providing a switched connection between a plurality of telephone extensions, whether via a private exchange or the public telephone network, it is necessary for the connection to be established via a so-called conference bridge. Since the speech signal originating from any one party to the conference call becomes attenuated as a result of the connection of the telephone line to the conference bridge and the consequent loading of the line by the plurality of parallel-connected lines of the other parties to the conference, known conference bridges incorporate at each port to the bridge at which a connected telephone line is terminated, a so-called conference port amplifier, the purpose of which is to provide compensation for attenuation of a signal applied to the bridge, either due to the loading presented by the conference bridge itself, or due to the attenuation arising in the telephone line itself. Since such a conference port amplifier generally provides amplification for signals transmitted in both directions via the telephone line, it is necessary for stable operation of the amplifier to ensure that the loop gain of the amplifier is kept below unity. One approach to this problem is to utilise so-called directional amplifiers, wherein at any given time amplification is provided in only one direction of signal transmission, while signals in the return direction are attenuated. In hitherto known types of directional switching amplifier, the action of this is usually arranged in the form of an electronic see-saw balanced to a mid-point when there is neither incoming nor outgoing speech. Switching of the amplifier to one or other direction is achieved in response to detection of the direction of the predominating signal applied to the amplifier, so that the amplifier is switched to transmit in the direction of the signal of greater amplitude. Such an arrangement has a number of disadvantages, and in particular, it prevents the amplifier from being switched to transmit an outgoing signal from the conference bridge to a telephone line connected at an individual port, in the face of a relatively greater signal transmitted from the telephone line towards the conference bridge.